1. Field
A high-temperature regenerative air heater transfers heat from a heat-surrendering combustion product or waste gas stream to a heat-absorbing air stream by alternate exposure of these streams of gas and air to heat-transfer members within the regenerative air heater. Of particular interest herein are methods for reducing stress corrosion cracking which occurs in metallic portions of a high-temperature regenerative air heater.
2. State of the Art
One example of a high-temperature regenerative air heater is a blast furnace stove. In a typical blast furnace iron-making operation, a furnace containing a charge of iron-bearing material, coke and flux is provided with a current of heated air, or "blast", which burns the coke. Hot blast air is usually obtained by heating air in a blast furnace stove, three or more of which are usually connected in a parallel arrangement. Blast air is provided by a blast furnace stove which at times operates in a three-phase cycle. In a first, or "on-gas" phase, the stove is heated by burning a combustible mixture of gases within the stove. In a second, or "stand-by", or "bottled" condition phase, the heated stove is held ready to deliver up its stored heat to an incoming air stream. In a third, or "on-blast" phase, the stove delivers up its stored heat to an air stream, which when heated by the stove, provides blast to a blast furnace.
During the heating phase of blast furnace stove operation, blast furnace top gas, which has been cleaned and then enriched with coke oven gas, natural gas, or other enriching fuel, to increase its BTu content, enters the stove combustion chamber together with air and is combusted. Products of combustion then pass through the stove heat exchanger, or checkerwork, and deliver up heat to the checkerwork, and thereafter exhaust to the atmosphere. During a typical heating of 45 to 90 minutes, certain portions of the stove, such as the combustion chamber, dome and top checkers, reach temperatures of about 1400.degree. C. or higher. After the stove reaches its fully-heated condition, the flow of combustible gases to the combustion chamber is shut off, followed thereafter by shut off of the combustion air. If the stove is not immediately placed in the "on-blast" phase after shutting off the gas flow, the stove is usually maintained in a stand-by or bottled condition with gases bottled in the stove at about atmospheric pressure. A bottled stove may be on stand-by for 20 minutes to one hour, although stand-by periods of much greater duration are not uncommon when furnace operation is delayed.
During cyclic operation of a blast furnace stove, temperatures of various interior portions of the stove may vary from ambient to above 1400.degree. C. These wide variations in temperature subject the stove walls and linings and other interior parts to repeated stresses. Stove walls are particularly susceptible to stress inasmuch as very high temperature gradients are created within wall portions, with the interior refractory lining exposed to temperatures of over 1400.degree. C. while the exterior, metallic jacket has a temperature normally ranging from about 50.degree. C. to 150.degree. C.
In addition to high stresses resulting from wide variations in temperature, the interior of a blast furnace stove shell is subjected to corrosive action of acids derived from nitrogen-containing oxide and sulfur-containing oxide gases formed during the period of combustion. Such gases include nitrous oxide (N.sub.2 O), nitric oxide (NO), nitrogen dioxide (NO.sub.2) and nitrogen tetroxide (N.sub.2 O.sub.4), all of which are generally designated as "NO.sub.x " gases. These gases may combine with water vapor formed by combustion in the first phase heating period or with water condensate formed on cooler areas of the stove walls, to form nitrous acid or nitric acid. Similarly, gases such as SO.sub.2 and SO.sub.3 may form sulfurous or sulfuric acids within the stove. It is well known that higher temperatures greatly favor the formation of nitrogen-containing oxides. As described in a recent German publication [Stahl and Eisen, 97(13), 633-637, (1977)], the highest concentrations of nitrogen-containing oxides develop during the bottled phase.
Metallic portions of stove walls are especially susceptible to attack from these NO.sub.x -derived corrosive acids. For example, the refractory lining of a stove is not typically gas tight, so that nitrogen-containing gaseous oxides can penetrate to the metallic jacket and react with water contained in the combustion gas or condensate found upon the jacket inner surface to form the aforementioned corrosive acids. Within portions of the metallic jacket where intercrystalline stresses are high, such as at seams, bends, creases, or weld points, the corrosive action of acids may be accelerated and may lead to cracking of the jacket. This phenomenon, known as "stress corrosion cracking", may require operation of the stove at reduced pressures and temperatures, and may cause ultimate structural failure of the stove jacket.
There are several blast furnace stove constructions known which are directed toward minimizing stress corrosion cracking. In U.S. Pat. No. 4,003,695 to Kandakov, for example, the inner surface of the stove metallic jacket may be coated with acid-resistant paints or gunite materials, or coated with "shotcrete" applied mortar-like compounds. The jacket, itself, may be fabricated of corrosion-resistant steel or layered with a foil of corrosion-resistant material. Other constructions utilize alternating layers of insulating materials and corrugated sheets to isolate the metallic jacket from the gaseous composition of a bottled stove.
All of these constructions suffer from reliability problems inasmuch as protection of the metallic jacket from the corrosive effects of the gaseous composition depends upon attaining gas-tight seals. Moreover, these complicated wall constructions are costly to fabricate for new stoves and are very difficult to install on existing stoves. Over a period of time, repeated cycles of large temperature variations cause differential contraction and expansion of stove wall components which result in overheating of bonding materials or coatings, or cracking of metal foil linings. Ultimately, the integrity of a gas-tight seal of even the more costly wall construction is lost.
There remains a need, therefore, for improved constructions and methods of blast furnace stove operation which alleviate conditions for stress corrosion cracking and which provide high reliability over the useful life of a blast furnace stove.